It is often desirable to have preprinted messages which can be revealed upon a predetermined stimulation such as pressure or abrasion. Typical uses are in game and lottery tickets where opaque, highly pigmented coatings are applied over printed messages. The opaque coatings are scraped off to reveal the message below. These systems tend to be quite messy, the abraded coating leaving particles and flakes that fall off the sheet.
U.S. Pat. No. 4,111,462 discloses an imaging system which comprises a coating of a colorless reactant which is dispersed in a binder with rupturable microcapsules. The microcapsules contain a second reactant which when contacted with the colorless reactant forms a visible color. By abrading the coating in an imagewise fashion, the microcapsules are broken, the reactants contact each other, and an image is formed.
Encapsulated materials have been used for many years in a wide variety of commercial applications. Early uses of encapsulated materials included paper coated with capsules bearing coloring material therein which could be used as a recording medium. U.S. Pat. No. 3,016,308 discloses one of the early efforts using encapsulated material as the image source on recording paper. U.S. Pat. Nos. 4,058,434 and 4,201,404 show other methods of application of encapsulated coloring materials on paper substrates to be used as imaging media and the like. U.S. Pat. No. 3,503,783 shows microcapsules having coloring material therein which are rupturable by the application of heat, pressure and/or radiation. The color is formed because of a metal coating on the surface of the capsule. These rupturable microcapsules, in one embodiment, may be secured between a substrate and a photoconductive top coat to enable photosensitive imaging of the system.
U.S. patent application Ser. No. 899,424 filed Aug. 22, 1986 in the name of Keith E. Relyea discloses a substrate that has a discontinuous coating of a material capable of reacting to form a color or be bleached to a colorless state. A coating of a binder with microcapsules is coated over the discontinuous coating. The microcapsules contain a reactant which will cause the color change in the discontinuous coating. Abrasion, pressure, or removal of a layer adhered to the binder layer releases the reactant to cause the color change in the discontinuous coating.